1. Field of Invention
This invention relates to a vibrating type pressure measuring device, wherein the natural vibrating frequency of a vibrator beam is changed according to the force applied thereto; and more particularly, to improvements in such a device.
2 Description of the Prior Art
FIGS. 1 to 4 are diagrams for explaining a conventional vibrating type pressure measuring device, such as, for example, disclosed in U.S. Pat. No. 4,841,775, wherein FIG. 2 depicts an enlargement of part A of FIG. 1 with a vibration detecting circuit connected thereto; FIG. 3 depicts section A-A of FIG. 2; and FIG. 4 depicts an electrical equivalent circuit of FIG. 2.
Turning to FIG. 1, the pressure measuring device comprises a substrate 10 of a silicon single crystal of D conduction type, wherein the upper surface has a crystalline plane (100), for example, of impurity concentration of less than 10.sup.15 atoms/cm.sup.3. On one surface of substrate 10, there is formed a diaphragm 11 which is engraved by etching from the back surface of a thin thickness.
The thick part 12 in the circumferential part of diaphragm is adhered to a pedestal 14 having a pressure conducting hole 13 in the center. Pedestal 14 is adhered to a pressure conducting pipe 15 in such a manner as to communicate to pressure conducting hole 13. Pressure P to be measured is introduced into pressure conducting pipe 15.
On the surface of the side not subjected to the above-described etching of diaphragm 11, shown with symbol A, there is partially formed an n.sup.+ diffusion layer (shown in abridged form in the figure) of an impurity concentration of about 10.sup.17, and on one part of the n.sup.+ diffusion layer, is formed vibrator 16 in the direction of crystal axis &lt;001&gt; (see FIG. 2). In vibrator 16, the n.sup.+ layer and the D layer are processed by photolithography and underetching.
A magnet 17 is provided in a non-contact state and perpendicularly to vibrator 16 at about the central upper part of vibrator 16. An SiO.sub.2 film 18 is provided as an insulating film (see FIG. 3).
Metal electrodes 19a, 19b, such as of aluminum, are provided. A terminal of electrode 19a is connected to the n.sup.+ layer elongated from vibrator 16 through a contact hole 20a provided via the SiO.sub.2 layer. Another terminal of electrode 19a is connected to a terminal of a comparative resistance R.sub.c, which has approximately equal resistance value to that of vibrator 16, and to the input terminal of an amplifier 21, respectively. The output terminal of amplifier 21, from which output signals are obtained, is connected to a terminal of primary coil L.sub.1 of a transformer 22. Another terminal of coil L.sub.1 is connected to common.
On the other hand, another terminal of comparative resistance R.sub.c is connected to a terminal of secondary coil L.sub.2 of transformer 22. The midpoint of secondary coil L.sub.2 is connected to common. Another terminal of secondary coil L.sub.2 is connected to a terminal of electrode 19b formed on another terminal of vibrator 16 via contact hole 20b of the n.sup.+ layer.
When reverse vias is applied between the D type layer (i.e. substrate 10) and the n.sup.+ type layer (i.e. vibrator 16) to insulate them, and DC current i is passed to vibrator 16, although the impedance increases in the resonance state of vibrator beam 16, the equivalent circuit shown in FIG. 4 is obtained when the impedance at this time is denoted as R.
Thus, since a bridge is formed with secondary coil L.sub.2, in which the midpoint C.sub.o is connected to common, the comparative resistance R.sub.c, and the impedance R, the non-equalibrium signals of this bridge are detected by amplifier 21. When the output of amplifier 21 is positive, the output is returned to primary coil L.sub.1, and the system generates self exciting vibrations at the natural frequency of vibrator 16.
The impedance R of vibrator beam 16 increases at the natural frequency. Impedance R is represented by the following: EQU R.apprxeq.(1/222).multidot.(1/(Eg.gamma.).sup.178 ).multidot.(AB.sup.2 l.sup.2 /bh.sup.2).multidot.Q+R.sub.d
wherein,
E=elasticity PA1 g=gravitational acceleration, .gamma.=density of material constituting the vibrator, PA1 A=a constant determined by the vibrating mode, PA1 B=magnetic flux density, PA1 l=length of vibrating beam, PA1 b=width of vibrating beam, PA1 h=thickness of vibrator beam, PA1 Q=sharpness of resonance, and PA1 R.sub.d =DC resistance value.
According to the above equation, since Q of vibrator 16 takes the value of several hundred to several tens of thousands, large vibration signals can be obtained as the output of amplifier 21 in the resonance state. As described, when the vibrating type transducer is constituted in such a manner that it carries out positive return by taking the gain of the amplifier 21 sufficiently large, the system carries out self exciting vibration at the natural frequency.
The vibrator may be one using p type material, for example, by diffusing boron into an n type silicon substrate for more than 4.times.10.sup.19 atoms/cm.sup.3 and by means of selective etching.
However, disadvantageously, in the device, when the resonance frequency of the diaphragm is in the performance frequency range of vibrator 16, and overlaps the resonance frequency of vibrator 16, such as by the change of pressure, lock-in is generated thereby to deteriorate the linearity, or hysteresis is generated.
Also, for measuring shock-like pressure, a squeeze R must be provided in pressure conducting hole 13 and a suitable time constant must be provided by adjusting the volume C of the pressure measuring chamber around the silicon diaphragm 11 so that destruction of silicon diaphragm 11 by the shock-like pressure, is prevented The word "squeeze" describes an absorber.
In general, since diaphragm 11 has resonance characteristics, it is designed in such a manner as hereinafter described.
First, FIG. 5 depicts the frequency characteristics of silicon diaphragm 11, wherein the ordinate represents dB and the abscissa represents frequency. The symbol f.sub.o denotes the resonance frequency of silicon diaphragm 11, and the symbol G.sub.o denotes the amplitude ratio at the resonance point.
Then, FIG. 6 depicts the frequency characteristics of the pressure conducting line, wherein the ordinate represents dB and the abscissa represents frequency. Symbol f.sub.c denotes the shut down frequency of the pressure conducting line, and symbol G.sub.L denotes the amplitude ratio, C the symbol C denotes the volume, and the symbol R denotes the squeeze. The total of the frequency characteristics are shown in FIG. 7.
The pressure conducting line frequency characteristics are preferably designed so that G.sub.o +G.sub.L .ltoreq.0 dB. In this case, (1) The shut down frequency f.sub.c of the pressure conducting line becomes low, and the frequency response deteriorates; (2) A large squeeze R becomes necessary; and (3) The capacity C must be made large, and the temperature characteristics deteriorate.
Due to such circumstances, miniaturization of prior art devices becomes difficult.
The terminology "pressure measuring apparatus using vibratable wire" refers to a device for detecting the vibration frequency of the vibrator, and for detecting the frequency output as a digital output. Since it is a digital output, it has high resolution and high S/N ratio. However, since such prior art vibrators are made of metal, they have the defective property of generating drift and hysteresis.